In all applications, such as the reading laser source in CD laser technology, minimization of the influence of optical feedback from external sources into the laser cavity is very important. The signal-to-noise ratio (S/N) in a laser device, depends on the coherent length of the laser beam L.sub.c, and the quantity of light feedback from the lens system to the laser cavity, in other words on optical feedback. Lasing occurs in the inner cavity mode and in the external cavity mode. The inner cavity mode is that which is achieved as a result of recombinations within the optical cavity resulting from injected carriers. The external cavity mode is occasioned by photons which reenter the cavity and experience collisions, thereby to alter the lasing signal in a manner which depends on the phase and frequency of the noise.
Turning first to the optical feedback aspect of the S/N problem, it is conventional to control the front facet reflectance in order to improve S/N. With low reflectance at the front facet (for example, at reflectance less than about 10%), the lasing mode of operation becomes the so-called multi-mode. At low values of optical feedback in that mode, the signal-to-noise ratio (S/N) is relatively high. However, as the optical feedback ratio increases (ratios greater than about 0.1%), S/N decreases because of the increase of the influence of optical feedback from sources external to the laser.
On the other hand, if the front facet reflectance is made relatively high (for example, greater than about 10%), the lasing mode becomes the so-called single-mode. With a high reflectance front facet laser, at intermediate optical feedback ratios (such as in the range between 0.01 to 0.1%), the mode competition noise increases substantially and as a result S/N decreases substantially compared to those characteristics at or near zero optical feedback ratio. At high optical feedback ratios, the lasing mode switches to multi-mode operation, and the signal-to-noise ratio increases to approach the S/N ratio achieved at zero optical feedback.
In summary with respect to control of facet reflectance as a means of controlling the signal-to-noise ratio, it is seen that such control is possible, but there are limits to the degree of control which can be exercised.
With respect to the second factor noted above which affects the signal-to-noise ratio, the coherent length of the laser beam L.sub.c also has an effect. It has been found that if the laser operates in the self-pulsation mode, the coherent length is decreased, and such decrease in coherent length favorably increases the signal-to-noise ratio. Furthermore, the self-pulsation mode is useful in achieving a high and relatively flat signal-to-noise ratio (versus front facet reflectance), making the device particularly useful as the read laser source in CD laser devices.
An important problem presented in self-pulsation laser diodes is the production of such diodes at a relatively high yield (and corresponding affordable cost). This is due, at least in part, to the fact that for most practical materials, the separation of the laser layers, particularly the separation between the active layer and the current blocking layer, must be relatively greater than in single mode lasers and must be controlled fairly carefully. As will be described below, if the desired thickness is achieved, the laser will reliably oscillate in the self-pulsation mode, if it is not, the laser will not and must be scrapped. As will also be described in greater detail below, conventional laser fabrication techniques are not completely suitable for achieving the necessary thickness and thickness control of the layer separating the active layer from the current blocking layer in the laser diode in order to economically produce self-pulsation laser devices at high yield.
More particularly, FIG. 2 illustrates a conventional laser diode of the buried heterojunction type utilizing a central mesa bounded by a pair of current blocking layers which confine current flow, and therefore lasing action to the central stripe below the mesa. In FIG. 2, there is shown a semiconducting substrate 1 comprising n-type GaAs which serves as the base for the semiconductor device. A lower cladding layer 2, preferably n-type Al.sub.0.5 Ga.sub.0.5 As is grown on the substrate followed by an active layer which is preferably p-type Al.sub.0.15 Ga.sub.0.85 As. A second cladding layer 4 comprising p-type Al.sub.0.5 Ga.sub.0.5 As is grown on the active layer. Subsequent to growth of the upper cladding layer 4, the device is masked and etched to produce a mesa generally indicated at 10 bounded by a pair of side regions 11, 12 from which the p AlGaAs material of the upper cladding layer has been removed. As shown in FIG. 2, the mesa has a width designated W and the upper cladding layer adjacent the regions 11, 12 is reduced to a relatively small thickness designated a. In the completed device, those parameters, when the diode is energized, cause lasing action in the region generally designated A.
Following etching of the mesa 10, the partly completed device is returned to the epitaxial growth apparatus and current blocking layer 5 of n-type GaAs is grown to fill the regions 11, 12 from which the p-type AlGaAs upper cladding material had been removed. Following growth of the current blocking layer, a p-type GaAs contact layer 6 is grown to complete the epitaxial growth process. Electrodes 7, 8 are then formed on the substrate and contact layers 1, 6, respectively, in conventional fashion to produce a completed laser diode. When a DC electrical potential is applied to the electrodes with the electrode 8 being maintained positive with respect to the electrode 7, and the current threshold is exceeded, carriers are injected into the active layer 4 in the region designated A to produce lasing action and generate coherent light which is delivered from the front facet of the laser. The current blocking layer 5 provides a reverse biased p-n junction to prevent current flow into the active layer in the regions covered by the current blocking layer, thereby confining the lasing action within the region A. In addition, the fact that the cladding layers have a higher aluminum content than the active layer creates a band gap discontinuity, with the higher band gap material being external to the active layer, thereby tending to confine the electrons and holes in the active region. The junction between the higher aluminum content cladding layers and the lower aluminum content active layer also produces a refractive index discontinuity (with the smaller refractive index in the cladding layers) which tends to confine the radiation (photons) to the active region. Thus, the cladding layers serve as a light guide to assure that the majority of photons generated in the active region are emitted at the front facet of the laser. More particularly, with respect to the photon confinement, a percentage of the light generated in the region A is absorbed in the n-type current blocking layer 5. This results in the production of an effective refractive index discontinuity in the transverse direction between the double heterojunction surface made up of the lower cladding layer 2, active layer 3 and second cladding layer 4, thereby to provide transverse mode stabilization.
Using conventional molar proportions of Al and Ga in the AlGaAs and typical carrier concentrations, a conventional structure as illustrated in FIG. 2 for single mode operation would utilize an active layer 3 having a thickness in the range of about 0.05 to 0.14 microns, a thickness a of the reduced portion of the upper cladding layer of about 0.2 microns and a width of the mesa of about 3.5 microns. Utilizing those dimensions, typically the device will exhibit a single vertical mode oscillation mode as is illustrated in FIG. 4. It is seen that the device oscillates at a particular wavelength to produce high intensity at that wavelength, but is incapable of oscillating at other wavelengths.
If multi-mode operation were desired, it would be necessary to alter the aforementioned dimensions, particularly by increasing the dimension a, i.e., the thickness of the reduced portion of the upper cladding layer, to about 0.4 microns, and making the stripe width W about 4.3 microns. When such a structure is realized, the vertical oscillation mode exhibits a self-pulsation behavior as illustrated in FIG. 5, where the laser can operate at a number of wavelengths at related intensities as demonstrated in that figure. In intermediate ranges of the dimensions a and W, such as the condition where a is about 0.3 microns and W is about 3.5 microns, the laser exhibits a multi-vertical mode characteristic where the laser can oscillate at certain wavelengths and not at others. It is important in practicing the invention to produce a laser which has only the self-pulsation mode of operation, and to produce such a laser reliably and at high yield in a production environment.
In the prior art structure, however, although the thickness of the reduced section of the upper cladding layer, i.e., the dimension a, can be controlled by controlling the etching process which produces the mesa, achieving the desired precision of control is difficult. Typically, the etching process has a precision of only about .+-.10%. If the thickness a is desired to be etched much below 0.2 microns, there is a danger of overetching and entirely removing portions of the upper cladding layer whereupon the etchant will attack the active layer, producing a device which must be rejected. If the goal thickness is too large, the .+-.10% tolerance of that layer number is often unacceptable. However, when the device is designed to allow for a thickness of about 0.2 microns for the dimension a, it is possible to check that dimension by optical interferometry, as is well known. In that technique, the device is illuminated with infrared radiation, and an optical instrument utilized to discern the color produced by the interference pattern, the color being related to the thickness a of the remaining portion of the upper cladding layer. It is relatively easy to discern the appropriate color or color change when the thickness a is in the neighborhood of 0.2 or 0.3 microns. However, when the thickness gets much beyond 0.3 microns, checking by optical interferometry techniques become difficult and less precise. Thus, in the condition where it is desired to produce a thickness a of about 0.4 microns to assure operation of the laser in the self-pulsation mode, optical interferometric testing of the etched layer will be inadequate, and the accuracy expected of the etching process will typically be unacceptable in producing an economical yield in a production process.